1. Field of the Invention (Technical Field)
The present invention relates to membranes for gas separation applications.
2. Description of Related Art
Note that the following discussion may refer to publications that due to recent publication dates are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications arerior art for patentability determination purposes.
Membranes for gas separation offer low energy requirements, low capitol costs, low maintenance, and facile operation. The market for gas separation membrane equipment has grown over the past 20 years to a multi-$100 million/year business. Separation of non-condensable gases such as nitrogen from air, carbon dioxide from methane, and hydrogen from nitrogen, argon, or methane represent 90% of this business. Furthermore, the chemical, refining and electric power industries have all identified membrane technology development as a high research and development priority. For example, the chemical industry seeks development of high temperature, chemically inert membrane materials suitable for hydrophilic compounds in dilute streams, and mixed organic/inorganic composite membranes. Refining, the most energy intensive industry in the U.S., spends about 40% of the energy it consumes on distillation processes. Refiners seek chemically resistant membranes developed for hydrocarbon separations with the goal of reducing energy consumption by 20%. In the power industry, high temperature membranes would be valuable in separating H2 from CO2 in shifted synthesis gas or in incorporation into a membrane catalytic reactor, improving the efficiency of the water-gas shift reaction. Additionally, carbon dioxide sequestration costs are estimated at $100-$300 per ton of carbon emissions avoided using present technology. The power industry hopes to reduce this cost to $10 per ton of carbon emissions avoided by the year 2015. Achieving this goal would save the U.S. trillions of dollars. High temperature membrane technology is key to realizing this goal. Finally, with the advent of the U.S. Government's Hydrogen Fuel Initiative (“HFI”), the demand for membrane gas separations will likely increase further.
In order to address these current and emerging separation needs, inexpensive materials with greater thermal and chemical stability than are commercially available today are required. Development pathways to date have included ceramic membranes, carbon membranes, high temperature polymer membranes, and zeolites. The most promising of these pathways are the ceramic membranes. For example, continuous films of zeolites suitable for selective gas separations are notoriously difficult to fabricate on a laboratory scale, much less on the scale required for an industrial gas separation application. Zeolite membranes have been implemented commercially for dehydration of organic solvent using pervaporation, a process that can tolerate larger imperfections in the membrane structure. Alternatively, carbon membranes are relatively easy to fabricate, but densify in the presence of humidity, lose selectivity in oxygen environments due to pore enlargement at the elevated temperatures of interest, and are not sufficiently mechanically stable. Good progress has been made recently in the development of high temperature polymeric membranes. Even so, their operational temperatures are still limited to less than 400° C. Compared to polymer membranes, ceramic membranes based on silica, alumina, titania and zirconia have demonstrated larger permeabilities and selectivities and are less susceptible to degradation of selectivity as the temperature of the operation is increased. Additionally, these materials are also typically brittle and difficult to fabricate defect free. Defects on the order of 30 A are large enough to reduce selectivity in gas separation applications to below acceptable levels.
Polymer derived ceramics is a relatively young research area. Kroke, E., et al., Materials Science and Engineering Reports, 26, 97-199 (2000); Liew, L., et al., “Fabrication of S:CN MEMS by Photopolymerziation of Pre-Ceramic Polymer,” Sensors and Activators A 95, 120-134 (2002). With this technique, new types of ceramic materials for high temperature applications can be processed at relatively low temperatures (compared to traditional ceramic fabrication methods; Bengisu, M., Engineering Ceramics, Springer-Verlag: Berlin (2001)). Additionally, because the ceramic takes the form of its polymeric precursor, one can fabricate ceramic structures of geometries that are not possible with traditional ceramics fabrication techniques. See, e.g., Patent Cooperation Treaty Publication No. WO 2004/065316 A2, “Polymer Derived Ceramic Materials”, to Bowman, C., et al. (Aug. 5, 2004).
The present invention involves production of ceramic membranes via pyrolysis of a *thin polymer film. These types of ceramic materials, e.g., Si—C—N, are not amenable to the traditional membrane fabrication techniques and thus, have not previously been formed into membranes. These ceramics will be quite thermally, mechanically, and chemically stable. They will also have useful gas selectivity and permeability, as one would expect based on their counterparts that are fabricated via conventional methods, thereby providing a significant advance in high temperature membrane technology.